Industrial furnace applications operating at high temperatures require large quantities of structural steel to support furnace enclosures consisting of dense refractories and because of the high heat capacity of the dense refractories, considerable heat energy is unproductively expended in establishing equilibrium conditions between the furnace enclosure and the industrial process occurring within the furnace. This difficulty has been to a large extent alleviated through the use of lightweight refractories such as insulating firebrick, insulating castables, and ceramic fibers with low thermal conductivities.
A similar disadvantage exists with the composition of existing kiln furniture which consists of a heavy refractory, with a density range of between 139 and 189 lbs. per cu. ft. and a chemistry ranging from 48 to 99% alumina. This kiln furniture is used in such applications as in the electronics industry where saggers are used to fire high alumina substrates for integrated circuitry, in the powdered metals industry where high alumina pusher tiles are used to heat powdered metal parts, and in the atomic power industry where a cast high alumina sagger is used to fire uranium dioxide. In the current energy crisis there is a need for a low mass, high alumina refractory having lower heat capacity for achieving savings in energy requirements in the various industrial processes utilizing kiln furniture without a sacrifice in the physical properties of the refractory.
U.S. Pat. No. 2,340,194 teaches a method of manufacture of strong, thick-walled alumina bubbles containing substantial amounts of other substances in addition to aluminum oxide for use in an insulating brick formed by a pressure operation. A pressed refractory would be unsuitable for kiln furniture where a smooth surface is essential. Further, the high weight percentage of alumina bubbles containing significant amounts of titania would not yield the necessary physical properties for use in kiln furniture. Slip casting, disclosed as an alternative to pressure forming of the brick, would also produce kiln furniture with inferior physical properties due to the necessity of using fine aggregates with moderate water conditions. Slip casting would be undesirable with a course aggregate mix due to segregation of the aggregate with the amounts of water required.
U.S. Pat. No. 2,965,506 discloses a composition consisting of alumina bubbles, tabular alumina and calcium aluminate, the calcium aluminate acting as a cementitious ingredient of the mixture which when mixed with water, forms an air-setting cement. This is basically a mixture which is shipped to a job site in dry form and then mixed with water to produce a cement for repair of furnaces and the like but would be totally lacking in the smooth surface requirements of kiln furniture.
U.S. Pat. No. 2,966,421 covers a foaming method to produce an insulating material using a phosphate bond to achieve the set. This type of system will not yield the surface quality or strength required for kiln furniture.